1. Field of the Invention
The present invention relates to a semiconductor laser array and more particularly to a phase-shifted multi-wavelength distributed feedback (DFB) semiconductor laser array for use in the multiple wavelength optical communication system.
2. Description of the Prior Art
A semiconductor laser utilized in the conventional multi-wavelength semiconductor laser array for use in the multi-channel optical communication system is called as a .lambda./4 phase-shifted DFB semiconductor laser and high in a single-mode quality, wherein the phase of a diffraction grating is shifted by a half period at the center of a laser resonance cavity, as illustrated in a cross-sectional view of FIG. 8. The .lambda./4 phase-shift structure is a known structure and is disclosed, for example, in "Semiconductor Laser", Japan Society of Applied Physics (ed.), Ohmsha Ltd., 1994, p. 272, FIG. 12--12. This structure is characterized by a capability to have a high side mode suppression ratio, since the laser oscillation wavelength therein is equal to the Bragg wavelength determined by the periodicity of grating thereof.
Further, in Japanese Patent Application Laid-open No. 109388/1987, a multi-wavelength laser array structure is described, in which a plurality of semiconductor lasers with each own grating periodicity are arranged in parallel. In this structure, each laser, having an individual electrode, is driven to emit a separate oscillation wavelength so that the laser array, as a whole, can be operated as a multi-wavelength semiconductor laser array.
However, in such a semiconductor laser array structure, the grating periodicity in each semiconductor laser is required to be formed to differ from each other so as to operate as a multi-wavelength laser array. In case that gratings are fabricated by electron beam exposure method using electron beam exposure apparatus, the individual grating periodicity must be set up with each laser and the grating are scribed one by one. It takes a longer time to form patterns of grating than in holographic exposure method. This causes a problem that this method is less suitable for mass production because of its too time-consuming exposure step.
Further, there is the second problem that, because a .lambda./4 phase-shift structure is located at the center of the laser cavity in the conventional .lambda./4 phase-shifted DFB semiconductor lasers, photons accumulate in the phase-shifted regions. This gives rise to a extremely non-uniform intensity distribution of the internal electric field and, consequently, a large disparity in the refractive index change over locations along the cavity caused by carrier movements, and thereby resulting in a severe fluctuation of the wavelength at the time of modulation.